The present invention relates to a field emission cold cathode device and a method for driving the same, and more specifically to a field emission cold cathode device having an emitter and a gate electrode located in the proximity of the emitter, and a method for driving the same.
A field emission cold cathode includes a Spindt type having a conically sharpened emitter and a gate electrode having an opening on the order of sub-micron and formed in the proximity of the emitter so that a high electric field is concentrated on a tip end of the emitter so as to emit electrons from the tip end of the emitter in vacuum; a silicon cone type;
the type having an emitter formed of a material having a small work function and a gate electrode located in the proximity of the emitter so that electrons are emitted by applying a high electric field; and a surface conductive type having two electrodes separated from each other by a micro spacing so that when a current is flowed between the two electrodes, an electric discharge occurs in a spacing between the two electrodes, and when electrons collide against an opposing electrode, secondary electrons are emitted into vacuum.
On the other hand, an image display using the cold cathode device, for example, a field emission display (abbreviated to "FED" hereinafter) is so configured that phosphors of three primary colors (red, green and blue) are located to oppose the emitters in a vacuum space, so that emitted electrons are injected to the phosphor to cause the phosphor to emit light. Therefore, it is a self-light-emission display device, and therefore, the color characteristics does not change dependently upon a viewing direction.
In order to obtain a good image quality, it is necessary to control the brightness of respective pixels to a desired brightness in time and spatially. In the FED configured to cause the phosphor to emit light, the brightness is in proportion to the product of the amount "n" of emitted electrons injected into the phosphor constituting the pixel, and an accelerated voltage "Va".
In an ordinary case, if electron emission having a constant current amount I is generated, the brightness can be controlled by controlling an emission time "t". In this case, the brightness is in proportion to I.cndot.Va.cndot.t. In the FED, a plurality of emitters are collected to form a cold cathode device, and one cold cathode device is provided for each pixel corresponding to one color. Therefore, cold cathode devices of the number corresponding to the number of pixels are prepared, and individual emitters are controlled.
Incidentally, there is a method for applying an anode voltage to only a phosphor to be caused to emit light, so that the phosphor actually emits light. However, it is basically important to maintain the emission current of the cold cathode device at a constant in time and spatially.
For example, in the Spindt type, the emission current is controlled by controlling the voltage of the gate electrode positioned in. the proximity of the sharpened tip end of the emitter. The emitted current is determined by an electric field in the proximity of the sharpened tip end of the emitter and the work function of the surface of the sharpened tip end of the emitter, in accordance with the Fowler-Nordhiem equation.
The electric field is determined by a voltage applied to the gate electrode, a distance between the gate and the emitter, and the degree of sharpness on the tip end of the emitter. In the Spindt type, an opening of the gate electrode is formed to have a diameter of 0.5 .mu.m to 2 .mu.m. However, since the opening of the gate electrode is ordinarily formed by use of a light exposure process using a photoresist, the gate opening has a variation on the order of 10% from one emitter to another.
In addition, the degree of sharpness of the emitter tip end widely varies from one emitter to another. The work function of the surface depends upon a crystal orientation. The Spindt emitter is ordinarily formed by evaporation of molybdenum, but since the molybdenum is polycrystal, the crystal orientation on the emitter tip end cannot be controlled. Furthermore, the grain size on the emitter tip end reaches about 10 .mu.m at maximum. As a result, the diameter of the emitter tip end widely varies.
The FED uses a cold cathode device obtained by collecting about 1000 emitters for one pixel. However, since the above mentioned variation occurs in the cold cathode devices, brightness unevenness occurs in the display. Furthermore, emission varies from one display to another.
The work function of the surface is widely different from one material to another. In particular, the work function greatly depends upon a material adhered on the surface and an oxidation condition of the surface. For example, when the emitter material is molybdenum, the work function is on the order of 4.5 eV in a metal condition, but as reported by E. Bauer and H. Poppa, Surf. Sci., 88, 31 (1979), increases by 2 eV when the surface is oxidized.
As mentioned above, the work function of the surface depends upon the crystal orientation. In the Spindt emitter, the crystal orientation on the emitter tip end cannot be controlled.
In the surface conductive type, the voltage applied between the two electrodes is controlled so as to control the current flowing through the two electrodes. The current amount is determined by the width of the narrow spacing between the two electrodes and the thickness of the electrodes. The width of the spacing between the two electrodes is controlled by a heat treating temperature of the electrodes and the time, but variation on the order of 5% occurs, with the result that the emission current varies.
Furthermore, in the FED having a fine gate or so constructed to apply an anode voltage of not less than 300V, an electric discharge accidentally occurs from matters adhered to the gate or the electrode. In the Spindt type, since a distance between the emitter and the gate electrode is very small, an electric discharge often occurs between the emitter and the gate electrode.
In order to prevent the above mentioned problem, it is ordinarily adopted to insert a high resistance between the emitter and a current supply so as to suppress the discharge. Ordinarily, an amorphous silicon layer or a high resistance polysilicon layer is formed on a glass substrate. However, the emission varies because variation in the length, the film thickness and the electric property of the resistor layer.
On the other hand, JP-A-10-207416 proposes an approach for suppressing the variation in the emission current. If a positive voltage is applied to the gate electrode, negative ions such as a residual gas. in an apparatus for driving the field emission cold cathode devices are attracted to the emitter and absorbed to the surface of the emitter, with the result that the emission current is decreased. JP-A-10-207416 is intended to overcome this problem. However, JP-A-10-207416 does not prevent the variation in the emission current caused by variation in emitter devices, namely variation from a field emission cold cathode device to another.